Use root_path for redirect

[Jaapp-]

In Starlette, I'm using something like this:

auth_app = GitHubAuth(...)
app.mount('/admin', app=auth_app)

In this case, scope["path"] == "/" and scope["root_path"] == "/admin".
As a result the redirect cookie becomes /, but I'd like it to redirect to /admin/ instead.

This PR fixes that by prefixing the redirect path with scope["root_path"].